High Efficiency Video Coding (HEVC), currently under development by the Joint Collaborative Team on Video Coding (JCT-VC) formed by ISO/IEC Moving Picture Expert Group (MPEG) and ITU-T Video Coding Experts Group (VCEG), is a video compression standard projected expected to be finalized in 2012. Similar to previous video coding standards, HEVC includes basic functional modules such as intra/inter prediction, transform, quantization, in-loop filtering, and entropy coding. HEVC defines a Largest Coding Unit (LCU) for a picture that is then partitioned into Coding Units (CUs) that take the form of rectangular blocks having variable sizes. Within each LCU, a quad-tree based splitting scheme specifies the CU partition pattern. HECV also defines Prediction Units (PUs) and Transform Units (TUs) that specify how a given CU is to be partitioned for prediction and transform purposes, respectively. A CU ordinarily includes one luma Coding Block (CB) and two chroma CBs together with associated syntax, and a PU may be further divided into Prediction Blocks (PBs) ranging in size from 64×64 samples down to 4×4 samples. After intra or inter prediction, transform operations are applied to residual blocks to generate coefficients. The coefficients are then quantized, scanned into one-dimensional order and, finally, entropy encoded.
HEVC also incorporates a Most Probable Mode (MPM) coding scheme for performing intra prediction of luma samples. In general, HEVC includes up to 33 directional intra prediction modes (“Intra_Angular” prediction modes or “directions”) and, for a given PU being coded, intra prediction modes may be encoded based on previously-decoded neighboring PUs. For example, for a PU (or PB) of size N×N, a total of 4N+1 spatially neighboring samples may be used for prediction purposes. The Intra_Angular directional modes may be labeled by an index k ranging from 2 to 34 so that Intra_Angular[k] specifies a specific directional intra coding mode (e.g., Intra_Angular[26] corresponds to vertical intra prediction). The remaining, non-directional, intra coding modes are Intra_Planar (k=0) and Intra_DC where Intra_Planar (k=1) uses average values of two linear predictions based on four corner reference samples, and Intra_DC uses an average of reference samples for prediction.
In the conventional MPM scheme, three candidate intra coding modes for a current PU may be derived from the intra modes used in already coded PUs located adjacent to the current PU in the same picture. In general, if adjacent coded PUs are not available or not intra coded, then Intra_DC is chosen as the candidate mode for the current PU.
If adjacent, previously-coded PUs are available, are intra coded, then the candidate intra modes in the traditional MPM scheme correspond to a set of three specific intra mode candidates depending on the intra mode of the left adjacent PU (the “LeftMode”) and the intra mode of the above adjacent PU (the “TopMode”). If the above adjacent PU is from another LCU, the TopMode may be set to Intra_DC. If the LeftMode is the same as the TopMode, then the three intra mode candidates are designated as follows: if LeftMode is Intra_Planar or Intra_DC, then the three candidate intra modes are Intra_Planar, Intra_DC, and Intra_Angular[26] (vertical); otherwise the three candidate intra modes are LeftMode, LeftMode−1, and LeftMode+1. On the other hand, if LeftMode and TopMode are different, then the three intra mode candidates are LeftMode and TopMode and a third candidate mode designated as follows: if neither of LeftMode or TopMode is Intra_Planar then the third candidate mode is Intra_Planar; if neither of LeftMode or TopMode is Intra_DC then the third candidate mode is Intra_DC; if neither of LeftMode or TopMode is Intra_Planar or Intra_DC then the third candidate mode is Intra_Angular[26] (vertical).
HEVC is also expected to include a Scalable Video Coding (SVC) extension. An HECV SVC bitstream includes several subset bit streams representing the source video content at different spatial resolutions, frame rates, quality, bit depth, and so forth. Scalability is then achieved using a multi-layer coding structure that, in general, includes a Base Layer (BL) and at least one Enhancement Layer (EL). This permits a picture, or portions of a picture such as a PU, belonging to an EL to be predicted from lower layer pictures (e.g., a BL picture) or from previously coded pictures in the same layer. However, in the conventional MPM scheme, only intra modes of adjacent PUs of the same layer (e.g., EL) may be employed to determine candidate intra prediction modes for a current PU.